Grinding method and apparatus therefor



Aug. 22', 1961 E. A. THOMPSON 2,995,848

GRINDING METHOD AND APPARATUS THEREFOR Filed March 14, 1957 sSheets-Sheet 1 Fla.

INVEN TOR. [4 EL 4. THOMPSON BY ATTORNEYS- Aug. 22, 1961 E. A. THOMPSON2,996,848

GRINDING METHOD AND APPARATUS THEREFOR Filed March 14, 1957 3Sheets-Sheet 2 O Q *2 x ZR o "Q.

N 1 g g a t Z or i k INVENTOR. [4R4 A. THOMPSON AT QRNEYS.

Aug. 22, 1961 E. A. THOMPSON 2,996,843

' GRINDING METHOD AND APPARATUS THEREFOR Filed March 14, 1957 3Sheets-Sheet 3 INVENTOR. H6 424. A. Tqompsorv A TTORNEYS 3 I f /WIfQWL;

2,996,848 GRINDING METHOD AND APPARATUS THEREFOR Earl A. Thompson, 1300Hilton Road, Ferndale Station, Detroit 20, Mich. Filed Mar. 14, 1957,Ser. No. 645,987 22 Claims. (CI. 51-95) This invention relates to agrinding method and apparatus therefor. More specifically, the inventionhas to do with a method and apparatus for rough and finish grinding workpieces in a single operation.

Finish grinding operations as conventionally performed are relativelyslow and tedious, and the time consumed in obtaining a desired finish bygrinding increases as the fineness of the finish desired increases.Thus, in a conventional grinding operation where it is desired to obtainan extremely fine finish, say 2 or 3 microinches, such a finish is onlyobtainable practically by sparking ou that is, by causing the piece tomake numerous passes across the face of the wheel without any relativeradial feed between the work piece and the wheel, except that producedby the relief of deflections in the machine and work, so as toeventually remove all the high spots from the finished face of the workpiece. This operation is slow for several reasons. In the first place,sparking out obviously consumes a lot of time. Furthermore, a finishgrinding Wheel is a relatively hard wheel; and unless the work piece isfed across the face of the wheel very slowly, burning or burnishing willresult.

It is an object of this invention to provide a method and apparatus forrough and finish grinding work pieces very rapidly in one operation.

A further object of the invention is to provide a method and apparatusfor grinding work pieces in a very rapid manner and at the same timeobtain a finish in the range of 1 /2 to 3 /2 microinches.

The invention contemplates a grinding machine wherein a rough grindingwheel and a finish grinding wheel are adjacently positioned on arotating shaft so as to rotate about exactly the same axis. The machineincludes a rotating spindle driving a work collet, the spindle andcollet being fixedly mounted on a support that is reciprocated in anaccurate predetermined path so that a work piece in the collet is causedto make a pass across the peripheral faces of the two grinding wheels.Means are provided on the machine for dressing the peripheral faces ofthe two grinding wheels so that as a work piece makes a pass across thefaces of the two wheels, the depth of cut of the finish grinding wheelis a very amount, just suflicient to produce the desired surface finishwith neglible stock removal. For this purpose the faces of the twowheels are dressed so that the finish wheel is only a very tiny amounthigher than the roughing wheel and preferably the finish wheel is one ofa higher degree of hardness than the roughing wheel.

In the drawings:

FIG. 1 is a side elevational view of a grinding machine in accordancewith the present invention as viewed from the operators side of themachine.

FIG. 2 is a top view of the machine.

FIG. 3 is a sectional view taken generally along the lines 3-3 in FIG. 2and showing the track arrangement for guiding the reciprocating movementof the table of the machine.

FIG. 4 is a sectional view generally along the lines 4-4 in FIG. 3 andshowing the motor means for reciprocating the table of the grindingmachine.

FIG. 5 is a generally diagrammatic view showing a manner in which thegrinding wheels may be dressed in accordance with this invention.

2,996,848 Patented Aug. 22, 1961 FIG. 6 is a cross section on line 6-6of FIG. 1 showing a portion of the wheel dressing mechanism.

FIG. 7 is a diagram showing the relative positions of the grindingwheel, the dresser bar, the work piece, and the work table guidearranged for grinding a spherical surface on the work piece.

FIG. 8 is a view corresponding to FIG. 7 showing the parts arranged togrind a flat surface on the work piece.

FIG. 9 is a cross sectional view on a very highly enlarged scale of awork piece partially ground in accordance with the present invention andillustrating in a schematic way the finish produced by the rough andfinish grinding wheels in a grinding arrangement according to thepresent invention.

FIG. 10 is a diagram showing portions of the rough and finish grindingwheels.

Referring to the drawings, and particularly to FIGS. 1 and 2, a grindingmachine is illustrated at 10 and includes an accurately journalled shaft12 on which is mounted a rough grinding Wheel 14 and a finish grindingwheel 16. On the base of the machine, there is mounted a hydrauliccontrol unit 18 of known construction for actuating the moving parts ofthe machine. On the bed 20 of the machine, there is mounted a table 22which supports a housing 24 in which is journalled a spindle 26 thatdrives a work collet 28. Spindle 26 has a belt drive from a motor 29.The work piece is designated 30. Table 22 is designed to be reciprocatedback and forth so that the work piece 30 is traversed once across theperipheral faces of grinding wheels 14 and 16 and returned out ofcontact therewith. The path of reciprocation of table 22 is determinedby a guide bar 32 mounted on the bed of the machine as by screws 34 andhaving an arcuate cam face 36. The cam face 36 may be a circular are asshown in FIGS. 2 and 7 or the cam face may be straight as shown at 36'on a guide bar 32 in FIG. 8 On the underside of table 22, there ismounted a pair of rollers 38 that engage the arcuate face 36 of guidebar 32. Rollers 38 are urged against the arcuate cam face 36 of guidebar 32 by a bearing pad 40 that engages a shoulder 42 formed on theunderside of table 22. Pad 40 is journalled as by a bearing 44 on a pin46 extending transversely through the end portion of a plunger 48.Plunger 48 is slideably arranged in a bore 50 formed in the bed of themachine and the plunger is biased in a direction away from the grindingwheels 14 and 16 by a coil spring 52 acting between a snap ring 54 inbore 50 and a washer 56 mounted at the free end of plunger 48. With thisarrangement, it will be observed that the table 2.2 is at all timesrigidly backed in a hori- U zontal plane against the guide bar 32 sothat there will be no tendency for the table to shift away from thegrinding wheels as the work piece is caused to make a path across thegrinding wheels.

The table 22 is arranged to be reciprocated through a predeterminedcycle by the power unit 18. The particular means. employed for impartinga reciprocating movement to table 22 are shown in FIG. 4 and include acylinder 5'8 pivotally supported on the bed of the ma: chine as by a pin60 and a double-acting piston 62 within cylinder 58 and having a rod 64that is pivotally connected to the underside of table 22 as by a pin 66.Passageways 68 and 70 are formed at the opposite ends of cylinder 5'8 topermit the admission of pressure fluid to the opposite working faces ofpiston 62 and thereby reciprocate table 22. Fluid for this purpose isdelivered from the power unit 18 to the opposite ends of a cylinder 58as required to produce the desired table movement. The extent ofmovement of table 22 in the feed direction is controlled by theengagement of an adjustable stop 72 on the table with a stop button 74on the bed of the machine. The position of the table in the re tractedposition is determined by an adjustable stop 76 on the bed of themachine that is engaged by a stop lug 78 mounted on the underside of thetable 22.

The particular cycle of the table as well as the manner in which thetable is actuated through its cycle by the power unit 18 preferablyincludes loading of the collet 28, unloading the collet, shifting thetable 22 from the load and unload position to the feed position, feedingthe work piece across the faces of the wheels and retracting the tableto the load position. The complete cycle is performed very rapidly bythe mechanism referred to and the actual feeding of a work piece, thatis, for ex ample, about 1 inch in diameter, across the faces of thegrinding wheels 14 and 16 consumes about 2 seconds.

In the arrangement illustrated, it is desired to grind the end of workpiece 30 with a radius, that is, the radius of the arcuate cam face 36of guide bar 32. However, whether the work piece is ground fiat or witha radius, a means is provided for dressing the faces of grinding wheels14 and 16 to the desired contour. In the arrangement shown, thedresseris shown at 80 and includes a diamond tool 82 carried by a holder84 reciprocably mounted in a casing 86. Casing 86 is guided on a base 88for traversing movement across the faces of wheels 14 and 16 and a camfollower member 90 on holder 84 engages a dressing bar 92 on base '88 todefine the path of the diamond 82 as it traverses the peripheral facesof wheels 14 and 16. The dressing bar 92 is clamped in the vise jaws 93in the customary manner. On the base 88 there is pivotally mounted across slide 95 (FIG. 6) having a pivot stub 97 received in a suitablebore in the cross slide 88. Suitable clamp screws 99 and a suitablescale and pointer 101 are provided for adjusting the angular position ofthe cross slide 95 with respect to the base 88. In the arrangement shownin FIGS. 2, 6 and 7, the diamond dresser is arranged to dress theperipheral faces of wheels 14 and 16 to a slight concave form, the wheel16 being dressed with a shoulder 94 (FIG. so as to grind a chamferaround the edge of the work piece. Wheels 14 and 16 are spaced slightlyapart so that there will be a clearance 96 therebetween, thus providinga very clear line of demarcation between the grinding faces of the twowheels, a line of demarcation that is readily perceptible to theoperator when the wheels are being dressed. Such a spacing is notessential however, and the two wheels may, if desired, be in directcontact or even adjacent sections of a single composite wheel. Theshoulder 94 may be formed as integral part of wheel 16 or it may beformed upon a separate wheel adjacent to it.

I have found that with an arrangement such as described herein, workpieces can be ground very rapidly and very smoothly if care is taken todress the two wheels relative to one another so that the depth of cut ofthe finish wheel 16 is a minimum amount.

By way of explanation, referring to FIG. 9, the portion 98 of thesurface of the work piece there illustrated is a highly magnifiedrepresentation of a surface produced by a rough grinding wheel 14. Forexample, with a wheel of 60 grit, the finish of surface portion 98 mightbe normally on the order of microinches. Now to obtain the smoothestpossible finish from the rough ground surface portion 98, the finishgrinding wheel 16 should be dressed so as to merely remove the highspots on the surface portion 98, so that the ridges 100 are cut downapproximately to the roots of the valleys 102. However, since the depthof these ridges on a 10 microinch finish is actually only 10 millionthsof an inch, it may be very difiicult as a practical matter to dress thefinish grinding wheel 16 so that it produces a step 104 on the workpiece as it is being ground of only 10 millionths of an inch.

Even though the height or diameter to which the finish grinding wheel 16is dressed is. largerthan the height or diameter of the rough grindingwheel 14 by an amount which initially may be somewhat greater than theoptimum step 104 on the work piece, it is possible to obtain anexcellent finish, however, without the step of sparking out. In otherwords, the cutting depth of the finish grinding wheel into the workpiece is so small that the finish wheel does not burn the work. Thiscondition is illustrated in FIG. 9 wherein the line 103 represents thesurface of the rough grinding wheel 14 when the finish wheel 16 isfreshly dressed to the proper height. The distance represented by thedimension 107 is the excess height of the finish wheel 16 over the roughwheel 14 when the former is freshly dressed.

The proper relation of the dressed heights of the two wheels can bereadily ascertained by running a colletheld work piece across the roughgrinding wheel 14 and partially across the finish grinding wheel 16slowly and without rotating the collet. When a work piece is ground inthis manner, the line of demarcation between the rough ground surfaceportion 98 and the finish ground surface portion 106 is defined by theshoulder 104 which can be measured with a very high degree of accuracywith present day high precision measuring instruments. When the stepproduced on the work piece is equal to approximately from one to fifteentimes the microinches of finish produced by the rough grinding wheel 14,then the finish grinding wheel 16 is suitably dressed to remove only thehigh spots or ridges produced by the rough grinding wheel in the shorttime during which finish grinding occurs. The result is that anextremely smooth surface is produced; and since the finish grindingwheel is cutting down a minimum of material, it is doing a minimum ofwork. Thus, the work piece can be fed across the face of the finishgrinding wheel at a relatively high rate without burning the work pieceand the leading corner of the wheel wears away at a relatively slowrate.

A preferred way to obtain the desired relationship in the finish andrough wheels is to utilize a substantially harder wheel for a finishwheel and to dress both wheels from the same dresser bar surface 92 withthe same diamond and follower. The extra hardness of the finish wheelresults in a higher depressed surface than that of the rough wheel eventhough the dresser bar 92 is not provided with a step at the pointcorresponding to the gap 96 between the two wheels. The additionaldeflections of the machine parts and the oil films in the journals issufficient to cause the finish wheel to dress out higher than the roughwheel due solely to its additional hardness. Thus, one satisfactorymethod is to dress both wheels by a single final pass of the diamonddresser 82 across the full face width of both wheels. Alternatively, ifthe finish wheel is left too high by this procedure, as will occur wherethere is a large differential in hardness between the two wheels, thediamond may be given a second pass across the finish wheel only,without, however, feeding the diamond inwardly for this purpose.

The height to which the finish wheel is dressed will vary for eachapplication and the optimum conditions can best be found by trial ofvariations in dressing method and in relative hardness of the twowheels.

Steel work pieces about an inch in diameter have been ground in thismanner using a 60 grit rough grinding wheel and a 320 grit finishgrinding wheel to obtain a finishof 1 /2 to 2 microinches in less than 2seconds.

With the wheels dressed to produce a shoulder '107 as shown in FIG. 9,about 100 millionths of an inch, a finish of 3 /2 microinches is readilyobtained. Preferably, as a practical matter, this step 107 is held atabout 50 millionths of an inch or less where a finish of 2% microinchescan be consistently obtained.

As illustrated in FIGS. 5, 6, 7, and 8, the finish wheel 16 ispreferably dressed with a shoulder 94. This is obassesses fi tained by acorresponding shoulder 118 formed on the cam bar 92. The purpose of theshoulder 94 is to provide an automatic breaking or slight charnfering ofthe corner on the finished face of the work piece without a separateoperation. In order to obtain this, the stroke of the table 22 isadjusted by means of stop 72, FIG. 4, to bring the edge of the workpiece into brief and limited contact with the shoulder 94 as theretraction of the spindle begins. This results in a small chamfer beingformed at the periphery of the finish-ground surface.

'It will thus be seen that the present invention provides an improvedapparatus for grinding work pieces in both a rough and finishingoperation in a single pass in the minimum amount of time. The provisionof both a rough and finish Wheel mounted on a common axis forsimultaneous or immediate successive engagement with the Work piece,without rechucking, results not only in a saving of time but in a higherdegree of accuracy than conventional methods. Furthermore, theparticular relationship of the working surfaces of the two grindingwheels results in an ideal functioning of the finish wheel, namely, toremove only the stock of the high portions left by the rough Wheelwithout removing significant additional quantities of metal notnecessary for the production of the desired high quality surface finish.In this way, the work pieces may be produced in a far shorter intervalthan heretofore accomplished with as high or higher quality of surfacefinish.

Thus other combinations of relative height and width of the rough andfinish Wheels may be selected and other dressing sequences be utilizedin order to maintain the desired small increment of height in the finishwheel above the rough wheel. If the wheels be selected of equalhardness, the rough wheel may be dressed down below the height of thefinish wheel by one or more extra passes of the diamond across the roughwheel only Without touching the finish wheel after the initial pass ofthe diamond across both wheels. The same procedure may be utilized whereit is desired to use a harder rough wheel than the finish wheel.

It will be understood further that the invention may be adapted to avariety of grinding machine configurations other than rotary facegrinding, such, for example, as internal and external cylindricalgrinding, centerless grinding, and in fact wherever a surface to beground is presented successively to the abrading surfaces of a rough anda finish grinding wheel, both rigidly mounted upon a common arbor andhaving the dressed wheel heights and the duration of finish grindingselected and limited in accordance With the principles hereinbeforedescribed.

What is claimed is as follows:

1. The method of rough and finish grinding hard metal work pieces in oneoperation which comprises rotating a pair of adjacently and fixedlypositioned rough and finish grinding wheels about the same axis, movinga work piece in an accurately predetermined path across the peripheralface of first the rough grinding wheel and then the finish grindingwheel and maintaining the faces of the two wheels dressed so that as thework piece moves along said path of travel, the depth of cut of thefinish grinding wheel is about to 100 millionths of an inch.

.2. The method of rough and finish grinding hard metal work pieces inone operation which comprises rotating a pair of adjacently and fixedlypositioned rough and finish grinding Wheels about the same axis, movinga work piece in an accurately predetermined path across the peripheralface of first the rough grinding wheel and then the finish grindingWheel and maintaining the faces of the two wheels dressed so that as thework piece moves along said path of travel, the depth of cut of thefinish grinding Wheel is about equal to the microfinish in inchesproduced on the work piece by the rough grinding wheel.

3. The method of rough and finish grinding hard metal work pieces in oneoperation which comprises rotating :1 pair of adjacently and fixedlypositioned rough and finish grinding wheels about the same axis, movinga work piece in an accurately predetermined path across the peripheraltace of first the rough grinding wheel and then the finish grindingwheel and maintaining the faces of the two wheels dressed so that as thework piece moves along said path of travel, the finish grinding wheel ishigher than the rough grinding wheel by a distance in the range of about50 millionths of an inch down to the microfinish in inches produced onthe work piece by the rough grinding wheel.

4. The method called for in claim 3 wherein the work piece is rotatedabout an axis generally perpendicular to the axis of rotation of thegrinding wheels.

5. The method called for in claim 3 wherein the work piece is moved inan arcuate path across the face of said grinding wheels.

6. The method of rough and finish grinding hard metal Work pieces in oneoperation which comprises rotating a pair of adj acently and fixedlypositioned rough and finish grinding Wheels about the same axis,mounting the work piece to be ground in a work holder, moving the workholder in an accurately predetermined path so as to cause the face ofthe work piece to be ground to be fed successively across the peripheralfaces of the rough and finish grinding Wheels and maintaining the facesof the two wheels dressed so that as the work piece moves along saidpath of travel, the finish grinding wheel is higher than the roughgrinding wheel by a distance in the range of about 50 millionths of aninch down to the microfinish in inches produced on the Work piece by therough grinding wheel.

7. Apparatus for rough and finish grinding hard metal work pieces in asingle operation comprising a solidly journalled rotatable shaft, arough grinding Wheel and a finish grinding wheel fixedly mounted side byside on said shaft, a work support, means for moving said work supportin an accurately predetermined path such that a work piece on said Worksupport is fed across the peripheral face of first the rough grindingwheel and then the finish grinding wheel, the peripheral face of thefinish grinding wheel being radially disposed outwardly relative to theperipheral face of said rough grinding wheel by a distance within therange of about 10 to millionths of an inch.

8. Apparatus for rough and finish grinding hard metal work pieces in asingle operation comprising a solidly journalled rotatable shaft, arough grinding wheel and a finish grinding wheel fixedly mounted side byside on said shaft, a work support, means for moving said work supportin an accurately predetermined path such that a work piece on said worksupport is fed across the peripheral face of first the rough grindingwheel and then the finish grinding wheel, the peripheral face of thefinish grinding wheel being radially disposed outwardly relative to theperipheral face of said rough grinding Wheel by a distance at leastequal to the microfinish in inches produced on the work piece by therough grinding wheel.

9. Apparatus for rough and finish grinding hard metal work pieces in asingle operation comprising a solidly journalled rotatable shaft, arough grinding wheel and a finish grin-ding Wheel fixedly mounted sideby side on said shaft, a work support, means for moving said worksupport in an accurately predetermined path such that a Work piece onsaid work support is fed across the peripheral faces of first the roughgrinding wheel and then the finish grinding wheel, the peripheral faceof the finish grinding wheel being radially disposed outwardly relativeto the peripheral face of said rough grinding Wheel by -a distanceWithin the range of about 50 millionths of an inch down to themicrofinish in inches produced on the Work piece by the rough grindingWheel.

10. The apparatus called for in claim 9 wherein said Work support isadapted to move said work piece in a plane parallel to the axis ofrotation of said shaft.

11. The apparatus called for in claim 9 including means for rotatingsaid work support about an axis generally perpendicular to the axis ofrotation of said shaft.

12. The apparatus called for in claim 9 wherein said means for movingthe work support in an accurately predetermined path comprise guidemeans fixedly mounted relative to said shaft and spaced generallyparallel therefrom, guide means on said work support engaging said otherguide means on the side thereof adjacent said shaft, at least one ofsaid guide means defining said predetermined path, means urging saidguide means on the work support against the fixedly mounted guide meansand means for reciprocating said work support in the path defined by theinterengaged guide means.

13. A combined rough and finish grinding apparatus comprising a machineframe, a grinding wheel arbor carried by the frame, means for drivingthe arbor, a work piece support also carried by the frame, means forcausing relative traversing movement between the work and the anbor, arough grinding wheel and a finish grinding wheel both rigidly attachedto the arbor and both presenting cutting surfaces to the work lying inthe path of traverse with the cutting surface of the finish wheel closerto the work by an amount which is of the same order of magnitude as theroughness of the work but not great enough to cause burnishing of thework piece and a common dressing means for both wheels in adjustablyfixed relation to the arbor.

14. A combined rough and finish grinding apparatus comprising a machineframe, a grinding wheel arbor carried by the frame, means for drivingthe arbor, a work piece support also carried by the frame, means forcausing relative traversing movement between the work and the arbor, arough grinding wheel and a finish grinding wheel, both rigidly attachedto the arbor and both presenting cutting surfaces to the work lying inthe path of traverse with the cutting surface of the finish wheel closerto the work by an amount which is of the same order of magnitude as theroughness of the work but not great enough to cause burnishing of thework piece the Width of the cutting surfaces along the path of traverseand the speed of the traversing means being correlated to limit thegrinding by the finish wheel to an interval sufiicient to removesubstantially only the ridges produced by the rough grinding wheelwithout removing a significant amount of the unground body of the workpiece and a common dressing means for both wheels in adjustably fixedrelation to the arbor.

15. A combined rough and finish grinding apparatus comprising a machineframe, a grinding wheel arbor carried by the frame, means for drivingthe arbor, a work piece support also carried by the frame, means forcausing relative traversing movement between the work and the arbor, arough grinding wheel and a finish grinding wheel, both rigidly attachedto the arbor and both preseting cutting surfaces to the work lying inthe path of traverse with the cutting surface of the finish wheel closerto the work by an amount which is of the same order of magnitude as theroughness of the work and not great enough to cause burnishings of thework piece the width of the cutting surfaces along the path of traverseand the speed of the traversing means being correlated to limit thegrinding by the finish wheel to an interval sufficient to removesubstantially only the ridges produced by the rough grinding wheelwithout removing a significant amount of the unground body of the workpiece and a single dressing tool having a traversing means for passingthe tool across the cutting surfaces of both wheels successively in asingle pass.

16. Apparatus for rough and finish grinding a hard metal work piece in asingle pass which comprises a solidly journalled unitary rotating shaft,an abrading device fixedly carried thereby and having side-by-sidesections of coarse and fine grit, respectively, with the fine gritsection slightly higher than the coarse grit section, a wheel dressingdevice mounted in adjustably fixed relation to the shaft and arranged todress the working faces of both sections in predetermined relation toeach other, means for causing relative motion between said device and awork piece along a fixed path parallel to the abrading surfaces and withno component of motion in a direction perpendicular to such surfaceswhereby the work piece is subjected to a major stock removing action inpassing onto and across the coarse grit section and to only a surfaceimprovement action requiring no sparkout in passing onto and across thefine grit section.

17. The method of simultaneous rough and finish grinding a hard metalwork piece which comprises rotating an abrading device with side-by-sidesections of coarse and fine grit, dressing the sections to slightlydifferent relative heights with respect to the work piece, and causing arelative motion between the work piece and the abrading device to removea major part of the grinding stock in passing onto and across the coarsegrit section and to merely improve the surface in passing onto andacross the fine grit section.

18. The method of rough and finish grinding a hard metal work piecewhich comprises dressing a multi-sectioned abrading device to apredetermined surface contour of the sections with respect to eachother, relatively moving the work piece and the abrading device along apath parallel to the surface of the abrading device first into astock-removing engagement with a coarse abrading surface and immediatelythereafter into a mere surface-improving engagement with a fine abradingsurface without in-feed motion in a direction perpendicular to theabrading surface.

19. The method of rough and finish grinding a hard metal work piecewhich comprises dressing a multi-sectioned abrading device to apredetermined surface contour of the sections with respect to eachother, relatively moving the work piece and the abrading device along apath parallel to the surface of the abrading device first into astock-removing engagement with a coarse abrading surface undersubstantial perpendicular force and immediately thereafter into a meresurface-improving engagement with a fine abrading surface while someperpendicular force is still maintained and without in-feed motion in adirection perpendicular to the abrading surface.

20. The method of grinding a hard metal work piece to size and finish ina single pass which comprises rotating an abrasive device havingside-by-side a rough grinding surface and a finsh grinding surfacedressed to a height a few millionths of an inch above the rough grindingsurface and causing relative movement between the work and the device ina direction parallel to said surfaces to remove stock by an interferingengagement with and a traverse across the rough grinding surface andduring the latter part of said relative motion improving the surfacefinish by traversing across the finish grinding surface without in-feedmotion in a direction perpendicular to the grinding surface.

21. Apparatus for rough and finish grinding a hard metal work piece in asingle pass which comprises a solidly journalled unitary rotating shaft,an abrading device fixedly carried thereby and having side-by-sidesections of coarse and fine grit and of different hardness,respectively, with the fine grit section slightly higher than the coarsegrit section, a wheel dressing device mounted in adjustably fixedrelation to the shaft and arranged to dress the working faces of bothsections in predetermined relation to each other, means for causingrelative motion between said device and a work piece along a fixed pathparallel to the abrading surfaces and with no component of motion in adirection perpendicular to such surfaces whereby the work piece issubjected to a mag'or stock removing action in passing onto and acrossthe coarse grit section and to only a surface improvement actionrequiring no spark-out in passing onto and across the fine grit section.

22. The method of grinding a hard metal work piece to a finish equal tothat obtained by a conventional spark-out operation, but in a time notsubstantially greater than that required for a rough grinding operationcomprising subjecting the Work piece to the immediately successiveaction of a coarse and a fine abrading wheel section both rigidlyrelated and rotating on a common axis with the surface of the finesection projecting beyond the surface of the coarse section by a smalldistance approximating the surface roughness left by the coarse section.

References Cited in the file of this patent UNITED STATES PATENTS WinnNov. 28, 1911 Sheehan Jan. 7, 1913 Busler May 18, 1915 Ames Oct. 16,1917 Hanson Mar. 6, 1923 Norton July 30, 1929 Raule Dec. 27, 1932 St.John July 7, 1936 Dunbar et a1. Apr. 23, 1940 Binns Dec. 10, 1940 GumperMar. 12, 1946 Holman et a1 June 6, 1950 Holman Feb. 13, 1951 FOREIGNPATENTS Germany Sept. 15, 1911

